1. Field of the Invention
This invention relates to a novel process for producing low valent Group VIII metal hydrides in which a low valent Group VIII metal halide complex is reacted with an alkali or alkaline earth metal alkylamide, containing at least one alkyl group having a hydrogen atom in beta position with respect to the metal atom.
2. Description of the Prior Art
Low valent Group VIII transition metal hydride catalysts are well known in the art as being useful in a wide variety of homogeneously catalyzed hydrogenation processes. For example, hydridotris(triphenylphosphine)rhodium(I), (Ph.sub.3 P).sub.3 RhH, is a well known catalyst in the homogeneous hydrogenation of olefins. Other processes in which the above catalysts as a class are known to be useful are the homogeneous hydrogenation of ketones, oximes, nitriles and activated esters, as well as in the isomerization, dimerization and oligomerization of olefins.
By the term "low valent Group VIII metals," as used herein, is meant that the metal atom in the Group VIII metal halide complex as well as the resulting metal hydride, containing iron, ruthenium or osmium is in the +2 valence state. For cobalt, rhodium and iridium metal halide complexes and resulting metal hydrides, the metal atom is in the +1 valence state. However, metal atoms of nickel, palladium and platinum are in the +2 valence state in the metal halide complex, but due to the instability of the resulting metal hydride, the resulting metal atom is transformed into the (O) valence state. However, in some cases the metal hydride may be stable.
The symbol "Ph" as used herein, designates the phenyl radical.
Methods of synthesis of low valent Group VIII transition metal hydrides include the use of aqueous ethanolic sodium borohydride in treating triphenylphosphine complexes of chlorides or chloro-anions of rhodium, ruthenium, iridium, osmium and platinum as described in J. Chem. Soc. (A), pp. 2947-2954 (1970). However, the disadvantages of the process are that inevitably boron halides are formed in the process which coprecipitate with the product methal hydride along with sodium borohydride reducing agent. Thus, the product is normally contaminated with these materials and usually difficult to purify. Also, excess sodium borohydride reducing agent is usually required to insure high yields in the process which adds to the total cost of the process.
The reference Inorg Chem., Volume 7, pp. 546-551 (1968), describes the preparation of hydrido phosphine complexes of ruthenium and rhodium by treating the complex metal chlorides with either hydrazine or aluminum trialkyls. The disadvantages of this process are that hydrazine presents a severe explosion hazard when exposed to heat and in contact with oxidizing materials. Further, aluminum alkyls are extremely toxic and flammable and have to be used with great caution.
The preparation of triphenylphosphine complexes of rhodium(I) hydride are described in Naturwissenschaften, Volume 56, pp. 415-416, and pp. 636-637 (1969), involving, for example the synthesis of tris(triphenylphosphine)rhodium(I) hydride by treating the corresponding chloride with potassium hydroxide in ethanol. However, the formed metal hydrides are exposed to a strongly alkaline medium in which decomposition of the formed product and reduction in yields are likely to occur.
The reference Inorganic Syntheses, pp. 121-123 describes the synthesis of tetrakis(triphenylphosphine)palladium(O) by treating palladium dichloride with triphenylphosphine and hydrazine. This process again has the disadvantage of requiring hydrazine as the reducing agent and also requires high reaction temperatures of about 140.degree. C.
The reference J. Chem. Soc., 1186 (1957) describes the preparation of palladium(O) complexes by reacting palladium isocyano complexes with triphenylphosphites and triphenylphosphine compounds. However, the disadvantages in these processes is that isocyano complexes are extremely toxic and the reaction doesn't have the general utility of preparing low valent Group VIII metal hydrides.
The references J. Am. Chem. Soc., Volume 95, pp. 3038-3039, (1973) and reference J. Org. Chem., Volume 41, pp. 2742-2746, (1976) describes the use of palladium black and palladium chloride, respectively in synthesizing unsymmetrical secondary and tertiary amines from substituted amines and converting secondary amines to enamines, respectively. Both references describe the possible participation of a beta-elimination process in producing the final amine products. However, the references do not describe the synthesis of useful low valent Group VIII metal hydride complexes which are not amine adducts.
The reference Chemical Communications, pp. 1274-1275 (1971) describes the synthesis of molybdenum amine-hydrides in water by reacting a dicyclopentadienyl molybdenum dimethylthiobromide cation with various amines. Excess amine and high temperatures are described as being necessary in the reaction in which it is postulated that a beta-elimination process may be operating, thus producing the cationic amine-hydride complex. However, the reference does not describe the synthesis of neutral low valent Group VIII metal hydrides useful in homogeneous hydrogenation processes which are not amine complexes.
Due to the wide range of utility of low valent Group VIII metal hydride catalysts the art in this field is constantly concerned with improving known methods for their synthesis particularly with respect to discovering new reducing agents which are not highly toxic, potentially explosive and are not required in excess in the synthesis process.